Mechanism And Process Of High-Temperature Copper Diffusion In Labradorite And Its Applications In Color Enhancement And Gem Identification

The natural sunstones occurred in Oregon,USA have high market values,because they not only exhibit the aventurescence of sunstones,but also can present the body colors of red and green like ruby and emerald at the same time.At the beginning of this century,a kind of artificially diffused red feldspar exhibited at the Tucson Exhibition in the United States.Its sellers claimed that this kind of red feldspar occurred in the Tibet of China,which attracted the attention of major gemological research institutions around the world.Since the artificially diffused red feldspars are very similar to the natural Oregon sunstones,the major gem research institutions have been working on the exploration of this kind of"red feldspar".However,because its color origin is the same as that of natural sunstone（copper nanoparticles）,and the details of the diffusion treatment process are less disclosed,there is no effective method to identify such gemstones at home and abroad.Therefore,it is of great significance to systematically carry out the research on copper diffusion treatment of feldspar and then establish an identification method for the copper diffusion-treated feldspars.In this thesis,the nearly colorless labradorite from Oregon was taken as the main research object.After collecting and sorting out the research results from domestic and foreign scholars,a novel method that the labradorite is partly buried in the diffusant was developed for the high-temperature copper diffusion process.Based on this method,many labradorite samples were treated by the high-temperature copper diffusion process.The red labradorite samples prepared in different diffusion conditions were systematically analyzed by the ultraviolet-visible absorption spectroscopy,fluorescence spectroscopy,laser ablation inductively coupled plasma mass spectrometry and transmission electron microscopy,so as to reveal the formation process and mechanism of copper nanoparticles in the labradorite.On this basis,the valence state control of copper in the high-temperature copper diffusion process,the surface color changing of faceted labradorite,and the identification characteristics of high-temperature copper diffusion-treated feldspar were further studied.The main results in this thesis are as follows:（1）This thesis takes the high diffusion rate of ions on the crystal surface as the basis of high temperature copper diffusion process,a novel method that the labradorite is partly buried in the diffusant was developed for the high-temperature copper diffusion process.The surface of the labradorite was successfully treated to red.Utilizing the surface of labradorite for high-temperature copper diffusion process can greatly shorten the research period,and only a small area of labradorite is in contact with the diffusant in the high-temperature copper diffusion process,so the surface of labradorite that is not in contact with the diffusant will not be contaminated,which greatly improves the quality of red labradorite treated by high temperature copper diffusion.（2）Based on the high-temperature copper diffusion process for labradorites being partly buried,the formation process and mechanism of copper nanoparticles on the shallow surface of labradorites are systematically studied in this thesis.The formation process mainly includes three key stages,stage I is the conversion of Cu⁺ions in the diffusant,stage II is the diffusion of Cu⁺ions from the diffusant to the labradorite through Cu⁺-Na⁺ions exchange,stage III is the reduction of Cu⁺ions to Cu⁰ atoms in the labradorite by capturing electrons inside the labradorite,and then the Cu⁰ atoms were supplied for the nucleation and growth of copper nanoparticles.The red labradorites obtained by high-temperature copper diffusion process were systematically characterized.The unique surface plasmon resonance absorption peaks of copper nanoparticles were observed in the UV-vis absorption spectra,which varied in the range of 575-586 nm due to the different sizes of copper nanoparticles.Two fluorescence emission peaks associated with Cu⁺ions（394 nm and 554 nm）exhibited in the fluorescence spectra of red labradorites under 320 nm excitation.The results of transmission electron microscopy showed that the particle size of copper nanoparticles formed in the red labradorite was46.0±5.9 nm,and the zero valence of copper nanoparticles in red labradorite was confirmed by electron energy loss spectroscopy.Many red cloudy and plume-like inclusions were observed on the shallow surface of the high-temperature copper diffusion-treated red labradorite under the optical microscope.（3）By introducing Li₂O into the diffusant,this thesis further developed a high-temperature copper-lithium diffusion process for labradorites being partly buried,which treated the surface of nearly colorless labradorite to blue.On this basis,the high-temperature copper-lithium diffusion process and coloration mechanism of blue labradorites were systematically investigated.The diffusion process mainly includes three key stages:stage I is the early diffusion of Li⁺ions to the labradorite through Li⁺-Na⁺ions exchange;stage II is the reaction of Li₂O with Cu O in the diffusant to form a more thermodynamically stable Li₂Cu O₂ phase with Cu²⁺ions at high temperatures;stage III is the diffusion of Cu²⁺ions into the shallow surface of labradorites at high temperature and the formation of a stable coordination structure with the pre-diffused Li⁺ions.The blue labradorites obtained by high-temperature copper-lithium diffusion process were systematically characterized.The typical broad absorption peaks of the chromogenic Cu²⁺ion near 750 nm were observed in the UV-vis absorption spectra of blue labradorites,and the fluorescence emission peaks at 394 nm and 554 nm associated with Cu⁺ions were absent in the fluorescence spectra.A large number of fractures distributed on the shallow surface of the blue labradorites were observed under an optical microscope,and a relatively severe fusion occurred in the labradorite area that is in contact with the diffusant.（4）From the perspective of gemological applications,the diffusion scheme that samples were partly buried in the diffusant was applied to the surface recoloring of faceted labradorite,and the spectroscopic and optical microscopic characteristics of the recolored faceted labradorite were characterized.The red and light-red faceted labradorites obtained by high-temperature copper diffusion process exhibited the unique surface plasmon resonance absorption peaks of copper nanoparticles near 580 nm,and the blue faceted labradorites obtained by high-temperature copper-lithium diffusion process exhibited the typical broad absorption peaks of Cu²⁺ions near 750 nm.The surface contamination and damage of the faceted labradorite recolored by high-temperature diffusion process were analyzed by an optical microscope,in which the blue faceted labradorite was seriously damaged in the area in contact with the diffusant,and the original facets were completely fused.The red and light-red faceted labradorite is less contaminated in the area in contact with the diffusant,which could be further reduced or eliminated by the optimizing the high-temperature copper diffusion process.（5）Based on the understanding of the formation mechanism of copper nanoparticles on the shallow surface of labradorites during the high-temperature copper diffusion process,the fluorescence emission peaks at 394 nm and 554 nm associated with Cu⁺ions in the fluorescence spectra were proposed as the key characteristic to identify the red labradorites treated by high-temperature copper diffusion in this thesis.The validity of the proposed key characteristic in fluorescence spectra was verified by introducing natural sunstones from the Oregon,USA and Afar region of Ethiopia as a comparison.On this basis,this characteristic in fluorescence spectra was applied to the identification of red andesine from Tibet by introducing the high-temperature copper diffusion-treated red andesine as a comparison.These red andesine samples from Tibet and the high-temperature copper diffusion-treated red andesine exhibited fluorescence emission peaks associated with Cu⁺ions（394 nm and 554 nm）under 320 nm excitation.